WATT PoultryUSA - December 2017 - 37
Relative Bioavailability Values and
John E. Thomson, Ph.D., Evonik Corporation
The value of feed ingredients in formulation
derives from which nutrients they deliver, the concentration of those nutrients and the ability of the
animal to use those nutrients for maintenance or
productive purposes. When making formulation
and buying decisions on ingredients, it is critically
important to know which ingredient delivers nutrients most economically.
Chemical assays identify which nutrients are
present and in what concentration but provide no
information on the availability of these nutrients to
the animal. Digestibility trials are commonly used
to determine amino acid and energy availability.
Combined with corrections for endogenous losses,
these techniques account for difference in digestibility but do not account for differences in metabolism
of the nutrients that may occur after absorption.
Minerals, vitamins and feed grade amino acids
typically serve as a concentrated source of one or
two nutrients. Formulating diets with high enough
concentrations of the single nutrient for use in a
standard digestibility trial is usually impractical due
to palatability or toxicity problems. Physiological
responses may also be affected by the unusually
Requirements for nutrients from simple sources
are often established using some common or "standard" additive. The availability of the nutrient from
the standard source is assigned a value of 100%.
Other sources are compared to the "standard",
and their availabilities are calculated relative to
the standard. Indexing of test ingredients versus
the standard ingredient is referred to as Relative
Bioavailability Value (RBV) or Bio-eﬃcacy (BE).
RBV is calculated as the amount of the standard ingredient (xs) needed to achieve a given response divided by the amount of a test ingredient (xt) needed
to achieve the same level of response
(RBV=(xs/xt) (Littell et al., 1995).
Comparing actual responses to the nutrients accounts not only for any differences in digestibility
but for differences in metabolism as well.
A linear response to a nutrient or ingredient can
be expressed mathematically as y= αx+ β, where
y is the response, α is the slope of the line, x is the
amount of the nutrient or ingredient, and β is the
response when none of the ingredient is added (yintercept). For the standard ingredient, we could
write this equation as ys= αs xs+ βs and, for a test
ingredient as yt= αt xt+ βt.
When comparing two source of the same nutrient, we are interested in the case where the responses are equal (ys = yt)
αs xs+ βs = αt xt+ βt
The response to no supplemental nutrient from
either source is the same and so, βs = βt and αs xs =
By rearranging the equations, we come to
So, RBV is equal to the ratio of the slope of the
response to the test ingredient to the slope of the response to the standard ingredient. We would set up
a trial with a basal diet deficient in the nutrient of
interest but at or above the requirement for all other
Figure 1. Relative bioavailability of Se from Selenium
Yeast (SY) versus 2-hydroxy-4-methylselenobutanoic
acids (SO) (Briens et al., 2014)
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